The present invention relates to computer controlled vehicle suspension systems and methods, and more particularly to vehicle suspension systems and methods in which computer controlled damping forces in compression and rebound directions are used to optimize ride and handling characteristics of the vehicle.
A typical suspension system interlinks wheels and axles of the vehicle with the body and chassis of the vehicle. The suspension system generally includes springs and damping devices. The spring compress and expand to minimize movement of the chassis and body when the wheels encounter perturbation in a roadway surface. Excess movement created by the springs is controlled by the damping devices.
A common damping device, or damper, is a velocity sensitive hydraulic system which uses hydraulic pressure to resist movement of a piston. Piston velocity is a direct function of the speed of the sprung mass with respect to the unsprung mass. As the damper is a velocity sensitive device, the greater the piston velocity, the greater the damping force provided by the damper in a direction opposite the movement of the piston.
The damping force is generally created when the moving piston forces a hydraulic fluid, typically oil, through an orifice or a valve. A flow resistance encountered by the oil results in a set of damping forces, a compression force and a rebound force. The damping forces act to counter and dissipate a stored energy associated with a springs movement that generates a spring-induced force. Varying the fluid flow through the valve or orifice varies the force acting against the spring-induced forces and, therefore, changes the ride and handling characteristics of the vehicle.
The damping forces are passive resistive forces. Accordingly, the respective compression and rebound forces only have effect when the piston is moving. When the velocity of the piston is zero no force is applied.
To improve ride and handling, one type of control system used in suspensions is a constant force type suspension system. Details of a constant force type suspension system are discussed in PCT Application entitled Computer Optimized Adaptive Suspension System and Method Improvements published Feb. 29, 1996, international publication number WO 96/05975 the entirety of the disclosure of which is incorporated herein by reference. In a constant force type suspension system pressures in the compression and rebound chambers of a damper are controlled by one or more pressure regulators to control a compression force and a rebound force. A computer utilizes a control input as a feedback signal to generate a constant compression and/or rebound force at a wheel.
If a vehicle equipped with such a constant force suspension hits a sharp bump the pressure is increased in a compression chamber of the damper. To maintain a constant force the control system allows for a pressure release of the hydraulic fluid from the compression chamber to maintain a constant force. This is done by providing a pressure relief valve on the regulator so that once a preset pressure is reached in the fluid, the valve opens to allow the hydraulic fluid to flow out of the chamber relieving the pressure. However, while a compression force is being applied and regulated it is not possible to regulate force in the rebound direction. Thus, only one chamber at a time is being pressurized, due to the piston movement.
A problem with this design is that there is a lack of force when the direction of travel of the damper is wrong for the application of a control force. For example, if during braking a bump is hit an upward force is exerted on the damper. After the wheel""s carrier assembly compresses to the maximum extent possible in one direction, it begins to rebound. During rebound the compression forces cannot be maintained and can only be reestablished once the wheel reaches its lowest position and begins to compress again. This is because the direction of travel has changed, and forces applied to control movement in a direction opposite to the movement are ineffective. Thus, control forces can be momentarily lost during damper movements in an undesired direction when attempting to maintain a constant force. This loss of force is not as serious as what happens when the direction of movement one again engages the control valve and the previously preset pressure returns very abruptly, which results in severe harshness. Mechanical compliance is a way of maintaining control forces during damper movement in an undesired direction.
FIG. 1 is a cross sectional view of a damper provided with mechanical compliance. Compliance is the ability of an object to yield elastically when a force is applied. In suspension systems compliance is provided by springs or rubber bushings in the damper. Compliance provides transitional forces during a period of time in which damper forces would otherwise be lost.
As shown in FIG. 1 compliance provided by a set of springs or a set of elastic rubber bushings has been incorporated in damper designs. A piston 102a is coupled to a damper shaft 104a. Resilient rings 106a and 108a are springs or rubber bushings that are disposed abode and below the piston, such that they elastically couple the shaft and piston. A bolt 110a and a washer 112a retain an upper resilient ring 106a to a damper shaft 104a. A lower washer 114a rests on a shoulder machined into the damper shaft 104a to retain a lower resilient ring 108a. The rubber bushings 106a, 108a compress when pushed upon.
Thus, when the damper shaft is forced upward hydraulic oil in compression chamber 116a pushes against the piston. Because the piston is not fixed to the shaft the rubber bushings or springs 108a are compressed. Accordingly, the movement of the damper shaft does not produce a corresponding equal movement of the piston. The oil in the compression chamber is in contact with the oil in an outer sleeve 118a through a port 120a. The outer sleeve is in contact with pressure regulator 122u through port 120a. Therefore, when the pressure in the compression chamber reaches a relief setting, pressure regulator 122a opens, allowing the fluid in the compression chamber to flow out of the chamber, maintaining a predetermined force on the fluid. Thus, the compliance provided by the resilient rings or springs 106a, 108a, allows the damper shaft to move upward before the preset relief setting of pressure regulator 122a has been reached. Therefore, instantaneous changes of piston position with respect for the chamber are allowed for.
In addition in some applications a loaded vehicle""s suspension becomes mushy. The ratio of a DFT of the SNF to one half of the SNF tracks the load change in producing compensating forces. In some SNF applications there was too little damping for small bumps and too much on big bumps. Taking the square root of the DFT SNF damping output provides optimum damping for all bumps. In providing UNF forces a steep force causes a mechanical strain on the system hardware. By taking the square root of the UNF damping force this is eliminated. In the bottoming out and toping out force generation sufficient control was not obtained. By taking position as a displacement and the velocity as an axle speed the responses now provide sufficient control.
After the springs become fully compressed as a result of the force applied from hitting the bump, the springs rebound in the opposite direction. Thus, the wheel and carrier move away from the chassis once the compressive force is removed. The lower rubber bushing 106a provides compliance in the rebound direction in a manner analogous to the discussion above.
The compliance provided by the mechanical system does not require the relief valves in the pressure regulators 122a and 124a to activate quickly when presented with short, abrupt motions. Further, compliance reduces high frequency vibrations and harshness encountered when the vehicle hits a bump by allowing the piston and chamber to move relative to each other, without, any delay due to control of the valves. As such, a suspension control system is desirable that is not subject to the problems of orifice controlled valves, and which incorporates valves that are not susceptible to the adjustment limitations and instabilities of current control valves. Thus, less expensive valves that tend to activate slowly may be used. However, the use of mechanical parts to provide compliance increases the number of parts required for fabrication and assembly.
The present invention accordingly provides a real-time computer control system for controlling one or more suspension control units (comprising damping devices) in a vehicle suspension system. The suspension control units control movement between sprung and unsprung masses in both compression and rebound directions with a process that uses constant force valves, replacing mechanical compliance with emulated compliance. Load compensation is provided to dynamically adjust ride and handling characteristics for changing load conditions. To achieve this the control system computes a signal for controlling each suspension control unit from position signals indicating the relative position (displacement) of the sprung and unsprung masses as related by the extension of the suspension control unit. Based upon the position signals (and other signals such as vehicle speed, acceleration, etc.), the control system interactively determines desired control forces to ensure optimum ride and handling characteristics. The ride handling characteristics affected are described by a set of parameters of motion (such as roll, pitch, body motions, etc.), emulated compliance, and load compensation. Processing of the set of parameters results in generation of a desired compression force and a desired rebound force.
The desired compression and rebound control forces for this set of parameters are determined, in part, by determining the amplitude of motion at the natural frequency of the sprung and unsprung mass systems. The natural frequency is isolated using an appropriate filter. Proportional damping forces in the direction opposite to the displacement of the spring from its normal or equilibrium position are determined and applied. A constant force is approximately maintained on the vehicle (emulating a constant force spring), thus minimizing body motions.
The process utilized to calculate the control forces utilizes the calculation methods described above to first provide preconditioning steps followed by an independent calculation of the parameters of motion. The resulting individual compression and rebound forces for each of the parameters of motion are summed to provide a resultant force in the compression and rebound directions to simultaneously control all of them.
In an embodiment the suspension control unit comprises a fluid control unit and a vehicle actuator. The fluid control unit is coupled to an actuator of a vehicle. Specifically, each actuator of a vehicle has a fluid control unit coupled to it. In forming a suspension control unit, each fluid control unit may be maintained separate from its corresponding actuator or may be integrated with its corresponding actuator into a single package.
Each fluid control unit comprises a reservoir which receives fluid displaced by the movement of the rod (and piston) in and out of the actuator. A position sensor and preferably a Linear Variable Inductive Transformer (xe2x80x9cLVITxe2x80x9d) position sensor is fitted within the reservoir to sense the volume of fluid within the reservoir. A microprocessor is coupled to the reservoir and receives signals from the position sensor for ascertaining the position of the actuator at any given time. An optional temperature sensor is also coupled to the reservoir for sensing the temperature of the fluid within the reservoir. The temperature sensor provides the microprocessor with the fluid temperature information so as to allow the microprocessor to ascertain the absolute position of the actuator by accounting for changes in the fluid volume due to temperature changes. Depending on the position of the actuator (and various other vehicle inputs), the microprocessor controls a pair of valves mounted on the reservoir for controlling the pressure of fluid entering or leaving the fluid control unit reservoir and thereby, controlling the pressure of the fluid entering or leaving the actuator and thus, controlling the damping provided by such actuator.
Each valve mounted on the reservoir comprises an annular body having a side passage and an end opening in communication with a passage on the reservoir, and a poppet slideably fitted within the body. The poppet can slide between a first seated position blocking the end opening of the valve body and a second retracted position not blocking the end opening. The poppet is moved into position blocking the end opening by a solenoid. A spring is used to slide the poppet back to a position not blocking the end opening when the solenoid is deactivated.
The poppet comprises a conical section and a cylindrical section extending from the larger diameter portion of the conical section. The conical section defines a tip portion of the poppet which is used to block the end opening of the valve body. The diameter of the largest diameter portion of the conical section is smaller than diameter of the cylindrical section. Consequently, an annular shoulder is formed extending radially around the poppet between the conical and cylindrical sections.
The conical tip section of the poppet is not exposed to the side passage when the poppet is in the seated position. As a result, the fluid pressure through the side passage is reacted against the poppet annular shoulder which is always exposed to the side passage whether the poppet is seated in the valve body or retracted from its seated position. Consequently, the fluid provides a force against the poppet annular shoulder tending to retract the poppet. A solenoid is incorporated that provides a variable force that tries to keep the valve seated in the closed position blocking the end opening of the valve body. As a result, the force set by the solenoid determines the pressure required to open the valve. Hence, the solenoid allows the valve to become an adjustable pressure regulator.
Since the area of the poppet annular shoulder exposed to the fluid pressure remains constant throughout the poppet stroke from a seated to a completely retracted position, the force generated by a given fluid pressure against the annular shoulder is constant tending to provide for a constant pressure regulation at different fluid flow rates. In other words, as the flow rate is increased, thereby increasing the fluid pressure, a larger force is reacted against the annular shoulder tending to retract the poppet further thereby canceling out the pressure created by the increased in fluid flow, thus, alleviating the instability problems associated with current valves incorporating poppets.
Moreover, applicant has discovered that a conical surface which is a section of a 70xc2x0 cone, i.e., a conical surface whose surfaces are tapered at 55xc2x0 relative to a plane perpendicular to the conical surfaces central axis, works optimally. This is because as the pressure on the poppet annular shoulder starts to open the valve, the fluid flow causes dynamic forces on the conical surface which would tend to close the valve (i.e., seat the poppet). However, as the poppet is retracted, the fluid pressure is reacted on a portion of the conical surface generating a retracting force as well as lateral force on the poppet. Applicant discovered that with the 55xc2x0 angle, the retracting force on the conical surface tends to cancel the dynamic flow force. This results in a constant pressure drop over wide ranges of fluid flow (e.g. 0 to over 50 gallons per minute)
The movement of the poppet is stopped when a flange extending from the poppet engages an inner annular shoulder formed on the valve body. As a result, the valve body is not loaded by the tip of the poppet as with conventional poppet valves alleviating the need to use expensive hardened steel as is used in the valve body of a conventional poppet valve for enduring the pounding by the poppet tip.
A spring biased check valve is slideably fitted around the body of each valve body to allow for flow out of the reservoir. When flow is tending to retract the poppet from its seated position, the check valve is closed and the valve regulates the fluid pressure. When the direction of flow reverses (i.e., the other valve is allowing flow to enter the reservoir) the check valve opens to allow the fluid to be bypassed back to the actuator. As the actuator pushes fluid back and forth through the fluid control unit reservoir, the pressures are correspondingly controlled in each direction as the fluid flows through one controlling valve and bypasses the other.
The fluid control unit and specifically the control valves of the present invention provide for a smooth metering of fluid to and from the actuators without the need for the large and expensive accumulators. Moreover, the fluid control unit of the present invention has a LVIT position sensor integrated into the reservoir. Furthermore, the valves of the present invention provide for better operational stability, are smaller and less costly, have longer fatigue lives and incorporate an optimum conical tip angle for providing a flat pressure response over different flow rates.